The present invention relates to a screw cutting control system which permits enhancement of screw cutting accuracy through utilization of a numerically-controlled machine tool.
In performing screw cutting using a numerically-controlled machine tool, it is customary in the prior art to carry out the following sequence with a fixed cycle. First, a tap 1 is positioned by a quick feed to a predetermined position in the X-Y plane of a workpiece 2 as shown in FIG. 1(A). Then, positioning of the tap 1 in the Z axis direction is effected by a quick feed while driving a spindle in a forward direction as shown in FIG. 1(B), after which, as shown in FIG. 1(C), the tap 1 is fed in the direction of the arrow at a feed rate dependent on the revolving speed of the spindle and the lead or pitch of the tap 1 while driving the spindle in the forward direction, thus performing screw cutting.
Upon completion of screw cutting of a predetermined quantity, i.e., depth, the rotation and feed of the spindle are stopped. However, the inertia of a spindle motor (not shown) is larger than the inertia of a feed motor (not shown) and even if stop commands are simultaneously applied, they do not stop at the same time but the spindle motor stops after the feed motor stops. Further, even after the feed is stopped, the tap 1 retains thrust if the spindle rotates and, accordingly, the tap 1 is coupled with the spindle through a tapper, or tap holder (not illustrated) for example, a tapper like those manufactured by Tapmatic Corporation. Next, as shown in FIG. 1(D), the spindle is reversed and, at the same time, the tap 1 is fed in the direction of the arrow at a feed rate dependent on the revolving speed of the spindle and the lead of the tap and when the tap 1 gets out of the workpiece 2, the tap 1 is returned by quick feed to a predetermined position as shown in FIG. 1(E).
Screw cutting by the above-described sequence has been accompanied by the following disadvantage. First, during screw cutting, the tap 1 must be fed by the feed motor. The feeding causes vibration which make it difficult to obtain a high degree of screw cutting accuracy. Second, when a stop instruction is received to stop the rotation and feed of the spindle after completion of a predetermined quantity or depth of screw cutting, the tap 1 is moved forward by the inertia force of the spindle motor to continue screw cutting even after the application of the stop instruction, therefore, it is difficult to obtain tapped holes of a designed fixed depth.